1. Field of the Invention
The invention relates generally to radio communication and more specifically to navigation receivers that operate with signals received from orbiting navigation satellites.
2. Description of the Prior Art
The global positioning system (GPS) supported by the United States government enables satellite navigation for military and civilian users alike. Two codes, a coarse-grained acquisition code (C/A-code) and a fine-grained precision code (P-code) are transmitted on two L-band microwave frequencies and each provide ranging and GPS-system time information. The C/A-code is available to civilian users and the P-code is normally available only to authorized users. During certain periods, the P-code is encrypted and such encrypted P-code is referred to as Y-code. The C/A-code is a digital sequence that repeats each millisecond and is unique to one of two dozen satellites. The P-code is a digital sequence that has a period of 269 days, with one week long segments of it transmitted intact. A single week-long segment is 10.23.times.10.sup.6 .times.604800 bits long, which comes from the P-code transmission rate of 10.23 MHz times the exact number of seconds in seven whole days. So a code phase uncertainty of even .+-.1 second can call for a search through 20,460,000 code chips.
Not knowing the exact global positioning system time is not an insurmountable problem in acquiring the code phase of the C/A-code, because a code generator in a GPS receiver can be slipped through all 1023 chips of the C/A-code in half-chip increments, and thereby discover the code phase in just a few seconds. Acquiring the P-code without knowing the precise global positioning system time is practically impossible. An external source of precise time, for example, coordinated universal time (UTC), is usually not available. The GPS satellite system time must be known to within much less than one millisecond, so the resulting code phase uncertainty is too great. A code-phase search can thus require searching through millions of chips for a match.
Therefore, authorized users tend to get to P-code code-phase acquisition and lock by first acquiring the code phase and then the global positioning system time available in the C/A-code. With knowledge of the global positioning system time, the P-code code-phase uncertainty is reduced to just a few chips.
The long length of the P-code provides so much processing gain in the associated correlators that the L1 or L2 carrier which carries the P-code is relatively hard to jam. The C/A-code on an L1 carrier is easy to jam, and since the codes transmitted are well-known, spoofing of the satellite transmissions is easy.
An authorized receiver that has been tracking P-code is handicapped, or totally prevented from re-acquiring the P-code if it loses its power supply in an uncontrolled manner, e.g., a battery goes dead or is unplugged, and the C/A-code on L1 is effectively being spoofed or jammed. Authorized receivers that are turned-off can go through a more graceful procedure wherein they save important data, e.g., global positioning system time and position.